Journal of Molecular Spectroscopy最新文献

The microwave spectrum of n-butyl nitrate was recorded in the 5 to 20 GHz frequency range using broadband chirp and narrowband pulse excitation molecular jet Fourier transform microwave spectrometers. A quantum chemistry structural analysis yielded thirteen stable conformers. Among them, the five most energetically stable conformers were observed in the experimental spectra. The most stable conformer features a butyl chain with an anti-gauche-anti conformation (AGA) where the γ-carbon atom is about 64° out of the nitrate plane. For this conformer, spectra of all ¹³C and ¹⁵N minor isotopologues could be measured. The conformer with a straight butyl chain (AAA), and three other conformers (GAA, GGA, and AGG) were also observed. Accurate rotational constants, centrifugal distortion constants, and ¹⁴N nuclear quadrupole coupling constants could be deduced and compared to the theoretical values.

Perfluoro-n-hexane and perfluoro-2-methylpentane are fully fluorinated alkanes used as non-ozone-depleting alternatives to chlorofluorocarbons and hydrochlorofluorocarbons. These compounds are long-lived and potent greenhouse gases due to their strong CF bonds and infrared absorption in the atmospheric window. Infrared absorption cross-sections are required to quantify the climate impact of these compounds via the radiative efficiency and global warming potential. To our knowledge, there are only two experimental measurements for perfluoro-n-hexane, and there are no experimental measurements for perfluoro-2-methylpentane in the infrared. In this work, we provide a set of absorption cross-sections in the range 515–1500 cm⁻¹, at 0.1 cm⁻¹ resolution from 298 to 350 K for each compound. We calculate the absorption cross-section between 0 and 515 cm⁻¹ using density functional theory with various basis sets. The 6-31,G(d,p) basis set with the B3LYP functional is found to provide the best results. Using both measurements and calculations combined, we calculate the radiative efficiency and global warming potential for each compound. No significant temperature dependence was observed in these quantities. The average radiative efficiency derived from all cross-sections is 0.48 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-n-hexane and 0.46 ± 0.06 W m⁻² ppbv⁻¹ for perfluoro-2-methylpentane. The average 100-year global warming potential derived from all cross-sections is 9590 ± 1260 for perfluoro-n-hexane and 9220 ± 1210 for perfluoro-2-methylpentane.

Calculations on the structure of 3,4,5-trifluorobenzoic acid were made using the Gaussian 16 program. The potential energy surfaces were scanned along CCCO and OCOH dihedral angles at the B3LYP/6-311G level to analyze its conformational landscape. Two conformations were identified and reoptimized at the B3LYP/aug-cc-pVTZ level. The result indicates that 3,4,5-trifluorobenzoic acid prefers a planar structure in its global minimum conformation. The pure rotational spectra of 3,4,5-trifluorobenzoic acid were measured in the frequency range of 6 – 12.5 GHz using a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer. The spectra of the parent, seven ¹³C, and one deuterium singly substituted isotopologues were analyzed and fitted to measurement accuracy for a semi-rigid asymmetric top molecule. The rotational constants and centrifugal distortion constants were accurately determined. The rotational constants for the parent isotopologue are A = 1535.31408(32) MHz, B = 650.31751(16) MHz, and C = 456.98499(12) MHz. The effective structure of its ground vibrational state was determined from the spectra of the mono-substituted isotopologues. The agreement between the calculated and experimental spectroscopic constants is excellent.

The microwave spectra of four carboxylic acid anhydrides, RCOOCOR′, (R,R′) = (CH₃, CF₃), (C(CH₃)₃, CF₃), (C₆H₅, CF₃) and (CH₃, C(CH₃)₃), have been observed in a supersonic jet. Calculations at the M06-2X/6-311++G(d,p) and MP2/6-311++G(d,p) levels of theory predict the lowest energy conformations for all four species to be nonplanar cis structures, i.e., conformations in which the C=O groups point in approximately the same direction, but are twisted out of a coplanar orientation. The observed spectra are consistent with these predictions. In addition, for all but the (R,R′) = (CH₃, C(CH₃)₃) species, higher energy nonplanar trans conformers are also predicted, typically within 1–2 kcal/mole the nonplanar cis form. For (R, R′) = (C(CH₃)₃, CF₃), extensive isotopic substitution has enabled a determination of most of the (non-fluorine) heavy atom structural parameters. Excellent agreement with the DFT and MP2 structures was obtained, thus validating the theoretical methods used. A strong correlation is found between the calculated O=C⋯C=O dihedral angle and the average of the vapor phase C=O stretching frequencies of RCOOH and R′COOH.

New measurements of the H${}_2$O-HF dimer high resolution absorption spectrum are performed using a relatively warm (240–260 K) equilibrium gas mixture with two complementary spectrometers: a video spectrometer and spectrometer with radio-acoustic detection. Positions of over hundred H${}_2$O-HF lines in the 158–345 GHz range are refined (their uncertainty is reduced by about order of magnitude) and several tens of lines are newly measured. These data are re-fitted together with previous measurements giving a refined set of constants of effective Hamiltonian which characterizes the dimer intermolecular dynamics. The new data improves significantly (from 100 to about 10 kHz) the accuracy of rotational constants corresponding to separate series of lines with fixed $K_a$.

We introduce two effective Hamiltonians that are suitable for analysis and fitting IR and MW high resolution spectra in non-planar or planar O₂-closed-shell complexes, where the closed-shell moiety is a diatom or a triatomic molecule of any symmetry. These new Hamiltonians differ in our choice for the direction of quantization of the projection of the electron-spin angular momentum. For both electron-spin coupling schemes, the total rotation-spin-tunneling Hamiltonians include tunneling, electron-spin–spin coupling, electron-spin-rotation interaction, and centrifugal distortion forces. In addition, we introduce the appropriate molecular symmetry treatment for an O₂ (${}^3{\Sigma }_g^{-}$)-XY₂ (e.g., O₂-SO₂ and O₂-H₂O) dimer in which the monomers exhibit permeation-inversion tunneling motion. Non-vanishing matrix elements of the total Hamiltonians and expectation values of six quantum numbers are evaluated in the appropriate basis set $\left(S,P_s;P,J,M_J\right)$. Diagonalization of the total Hamiltonian matrix provides the energy levels while the eigenfunctions are used to transform expectation values of the quantum numbers into the eigenfunctions basis and for calculation of relative intensities of the allowed transitions. The reported Hamiltonians were successfully applied for fitting the observed IR and FTMW spectra of the O₂-DF and O₂-SO₂ complexes, respectively.

The absorption spectra of carbon dioxide were recorded in the region from 15,000 to 15,300 cm⁻¹, using a Bruker IFS 125 HR Fourier transform spectrometer and a 30 m multipass cell with the White type optical system. The recording was performed at a spectral resolution of 0.044 – 0.050 cm⁻¹, room temperature, a path length of 1057.95 m and pressures of 185 and 362 mbar. Utilization of a LED as a light source provided a sensitivity (noise equivalent absorption) at the level of k_ν = 1.23 × 10^-10 cm⁻¹ and allowed detection of a number of lines of two 3005i − 00001 (i = 2,3) bands and several lines of the 60031 – 00001 band of ¹²C¹⁶O₂ with the intensity values down to 10^-30 cm⁻¹/(molecule cm⁻²) at 296 K. These bands were observed for the first time. The uncertainty of the line position measurements was estimated to be about 0.005 cm⁻¹ for the unblended lines with a high signal-to-noise ratio. The uncertainties of the retrieved line intensities for the strongest unblended lines are at the level of 15 %. The spectroscopic constants for observed bands were fitted to the observed line positions. The vibrational transition dipole moments squared of these bands were fitted to the observed line intensities. The measured line positions were compared to those from the HITRAN2020 database and to those predicted with the global effective Hamiltonian. The measured line intensities were compared to the values from the HITRAN2020 database and from the Ames2021 line list.

In our previous paper [K. Iwakuni et al., Phys. Rev. Lett. 117, 1439026 (2018)] the ortho-para dependence of the pressure effects was reported for the ${\nu }_1+{\nu }_3$ band of acetylene measured by dual-comb spectroscopy. Sears and coworkers [E.C. Gross et al., J. Chem. Phys. 154, 054305 (2021)] reevaluated the profile parameters for some lines of this band by comb-referenced laser spectroscopy, and pointed out that the ortho-para phenomenon must be a consequence of the use of Voigt function as the profile function, with the Doppler widths fixed at their theoretical values. Thus, we have remeasured this band with two improved spectrometers, the comb-locked laser spectrometer of the National Metrology Institute of Japan and another dual-comb spectrometer of Yokohama National University, and analyzed the lines by using the speed-dependent Voigt function as the profile function with several options. By the present work, we confirm the fact pointed out by Sears and coworkers, and present some remaining problems in the experimental determination of the line profile parameters.